Field of the Invention
The present invention relates to an objective optical system having a focusing function, and in particular, to an objective optical system for endoscope that enables a proximity magnifying observation, an objective optical system of a digital camera and a video camera that enables macro photography or an objective optical system of a miniature camera for consumer use.
Description of the Related Art
In a field of medical endoscopes, precise diagnosis of a lesioned part has been carried out by an observation through an endoscope (hereinafter, referred to as ‘endoscopic observation’). Therefore, an objective lens of an endoscope, in a state of being close to a lesioned part, is sought to enable magnifying observation of the lesioned part. As such objective lens, an objective lens in which it is possible to change an object-point distance (object distance) at which the focusing can be done (hereinafter, referred to as ‘magnifying endoscope objective lens’), is available.
In the magnifying endoscope objective lens, by focusing to an object point at a close distance, it is possible to carry out proximity magnifying observation. Moreover, it is possible to carry out normal observation by focusing to an object point at a long distance. The normal observation is an observation with a magnification lower than a magnification in the proximity magnifying observation. In the normal observation, it is possible to observe a wide range including a lesioned part and a surrounding portion thereof. As the magnifying endoscope objective lens, magnifying endoscope objective lenses disclosed in Japanese Patent Publication after Examination No. Sho 61-044283, Japanese Patent Application Laid-open Publication Nos. Hei 06-317744, Hei 11-316339 and 2000-267002 are available.
The magnifying endoscope objective lenses disclosed in Japanese Patent Publication after Examination No. Sho 61-044283, Japanese Patent Application Laid-open Publication Nos. Hei 06-317744 and Hei 11-316339 include a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At the time of focusing to an object point at a close distance and an object point at a long distance, the second lens group having a negative refractive power moves.
The magnifying endoscope objective lens disclosed in Japanese Patent Application Laid-open Publication No. 2000-267002 includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a negative refractive power. At the time of focusing to an object point at a close distance and an object point at a long distance, the second lens group having a positive refractive power moves.
In recent years, for improving an accuracy of diagnosis, a high quality of an endoscope image has been sought. In order to meet the requirement, a use of an image pickup element with number of pixels larger than the number of pixels of a conventional image pickup element has been started. Consequently, the magnifying endoscope objective lens is sought to have a high resolving power.
Moreover, in precise diagnosis of a lesioned part in recent years, it has been strongly sought that an observation of same level as an observation through a microscope (hereinafter, referred to as ‘microscopic observation’) be carried out even by the endoscopic observation. The observation of same level as the microscopic observation means an observation with about same degree of magnification and resolving power. For this, the magnifying endoscope objective lens is sought to have even larger magnification at the time of proximity magnifying observation. By doing so, an observation of same level as the microscopic observation is possible even by an endoscope.
In endoscopic observation, observation of a physiological tissue with the same level as the microscopic observation, such as an observation at the cellular level, has heretofore been difficult. Consequently, the only method available was to extract a part of a physiological tissue by biopsy after a lesioned part has been identified by endoscopic observation, and to observe the extracted physiological tissue by a microscope. However, by using a magnifying endoscope objective lens having a large magnification, there is a merit of being able to observe a physiological tissue with the same level as the microscopic observation while being in-vivo, as an extension of the normal observation.
As an endoscope objective lens which enables observation of the same level as the microscopic observation, endoscope objective lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2004-313769, 2004-313772, 2007-233036, 2007-260305 and 2009-294496 are available.
The endoscope objective lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2004-313769 and 2004-313772 are single focal length endoscope objective lenses. A single focal length endoscope objective lens is an objective lens in which an object-point distance to which the focusing can be done has been determined in advance. Therefore, the endoscope objective lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2004-313769 and 2004-313772 do not include a lens group that moves.
The endoscope objective lenses disclosed in Japanese Patent Application Laid-open Publication Nos. 2007-233036, 2007-260305 and 2009-294496 are magnifying endoscope objective lenses.
Two magnifying endoscope objective lenses have been disclosed in Japanese Patent Application Laid-open Publication No. 2007-233036. One magnifying endoscope objective lens includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At the time of focusing to an object point at a close distance or an object point at a long distance, the second lens group having a negative refractive power moves. Moreover, the other magnifying endoscope objective lens includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, and a third lens group having a positive refractive power. At the time of focusing to an object point at a close distance and an object point at a long distance, the second lens group having a positive refractive power moves.
The magnifying endoscope objective lens disclosed in Japanese Patent Application Laid-open Publication No. 2007-260305 includes four lens groups. At the time of focusing to an object point at a close distance and an object point at a long distance, a second lens group and a third lens group move, or the third lens group moves.
The magnifying endoscope objective lens disclosed in Japanese Patent Application Laid-open Publication No. 2009-294496 includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power. At the time of focusing to an object point at a close distance and an object point at a long distance, the second lens group having a negative refractive power moves.
Moreover, a desire to carry out the proximity magnification observation with a high resolving power and enlarge magnification is there not only in endoscopes, but also in digital cameras and video cameras. The macro photography in digital cameras and video cameras corresponds to the proximity magnifying observation. Therefore, even in optical systems of digital cameras and video cameras, high magnification and high resolving power in the macro photography have been sought. Moreover, in optical systems of digital cameras and video cameras, small-sizing of an optical system is desired.